In the last decade, tungsten-containing enzymes have been isolated from a number of microorganisms including hyperthermophilic archaea, mesophilic archaea, and bacteria. There is evidence that these are the most slowly evolving of all known archaea, and it has been speculated that these were the earliest life forms on earth. As such, the study of these enzymes could provide valuable insight into the evolution of enzymes and metabolic pathways. The purpose of this proposed research is to elucidate details of the structure and mechanism of tungsten enzymes using advanced EPR spectroscopic techniques. Electron nuclear double resonance (ENDOR) and electron spin echo envelope modulation (ESEEM) experiments will be carried out in order to characterize the physical and electronic structures of mononuclear tungsten enzyme active sites poised at various states during the catalytic cycle. [unreadable] [unreadable]